# Why do Leading Edge Flaps (LEFs) create lift while down at slow speeds, but decrease lift while down at higher speeds?

When taking off or landing, both LEFs and trailing edge flaps (TEFs) for fighter aircraft are in the down position to provide more lift. However, when flying around at higher speeds (let's say 500-600 knots), the left LEF going down and the right LEF going up causes the aircraft to roll left.

My personal theory is that the dominant form of drag (induced) at slow speeds means that the increased camber provides more lift although slightly decreasing the angle of attack (which is good if you are close to the stall angle of the wing). However, at higher airspeeds the slight decrease in angle of attack caused by the downward LEF and also the larger parasitic drag causes the decrease in lift of the wing.

Does this theory hold up? Or is there more to it that I'm not considering?

• Drag is drag, get rid of it as soon as it is not needed. Even in my 'litte' Cessna Cardinal, with 180 HP, flaps are retracted as soon the benefit of slow speed lift is overcome by sufficient lift being generated by higher climb/cruise speed. That's only 10 degrees on takeoff, just to help get over obstructions. After they are just drag. Back down again for landing for more lift at lower speeds, all the way to 30 degrees for touchdown. Apr 5, 2018 at 18:23
• 1. I don't know of ANY fighter aircraft with Leading-Edge Flaps. Slats yes, but not Flaps.
– RAC
Apr 6, 2018 at 8:25
• 2. >>the left LEF going down and the right LEF going up causes the aircraft to roll left<< This NEVER happens. Leading edge devices are always moved symmetrically.
– RAC
Apr 6, 2018 at 8:26
• @RAC F-16, F-18, to name a couple. Perhaps I should've worded the question differently. Say, for example, the left LEF of an F-16 gets stuck in the down position... The aircraft will have a left roll tendency for the rest of flight because the left wing is now producing less lift. Why, then, do we put LEFs in the down position when coming in for a landing? Is it to increase camber to allow the aircraft to maintain the same wing angle of attack at slower speeds without stalling? Apr 7, 2018 at 16:25
• Re " However, when flying around at higher speeds (let's say 500-600 knots), the left LEF going down and the right LEF going up causes the aircraft to roll left." -- the question would benefit from an explanation of how you know this to be true. Personal experience? Outside sources? Not saying it's implausible, but some of us would like to know how you know this to be true. But doesn't it make sense that at low AoA, the LE flap would just sort of act as a front-mounted aileron? Apr 1 at 15:30

Leading edge flaps, also known as slats, don't really create lift. The effect on lift coefficient is minor (less than 10% in general) but they help increase the stall angle of attack. As a summary, flaps creates lift by increasing surface area and profile camber, whereas slats postpone the stall and allow for higher AOA by reducing pressure drop around the leading edge. This increases the theoretical maximal lift by increasing the stall AOA but doesn't change much the lift coefficient.

Slats are ment to work at high AOA by design, at higher speed your AOA is very small, the slat is thus not working properly, disrupting the flow on the suction side of the wind and reducing actual AOA, thus reducing lift of that particular wing while simultaneously increasing drag.

• "Leading edge flaps, also known as slats" -- LEFs and slats are very different. // "disrupting the flow on the suction side" -- wouldn't up deployment cause more suction side disruption (re Q's roll example)?
– user14897
May 2 at 15:42

This site has some info on what the F-16 leading & trailing edges do during the flight - symetrically. We can see that at high speeds the wing is straight: it is at a low angle of attack, and at high speed the drag needs to be low.

When AoA is high, it needs the slats to be deflected, while at low speeds the flaps are deflected as well. The Approach configuration works as a thin wing with high camber.

The only reason I can think of is angle of attack. At low speed, angle of attack is higher, so even the downward drooping section ends up with a positive AoA, creating lift. At high speed, it is lower, causing the downward drooping section to have negative AoA, and thus create negative lift.

Now, why do we have this device in the first place then? (At least partly) Because a gradual curve, rather than a flat plate, results less drag and safer stall behavior.

When the leading edge flaps (slats) go down, they reduce the effective angle of attack, but allow increasing more by pitching the nose up. So

• When deployed down symmetrically, the plane can fly slower by increasing the angle of attack, assuming a nose higher attitude than it would otherwise be able to sustain for the same flight path.
• When deployed asymmetrically, the decrease of effective angle of attack on the side where it is down means overall lift decreases there—though the increase in camber increases the lift coefficient a bit, so the decrease is less than if the whole wing was rotated—and increases on the side where it is up—though again the negative camber reduces the lift coefficient a bit. That means it's not as efficient as using the trailing edge devices (ailerons) where the change in camber works in the same direction with the change of effective angle of attack.
• As a real world example of differential LEFs (DLEFs) as used by the F/A-18E/F: LEF range is -5° (i.e. up) and +34°, and in certain regimes they are used differentially for rolling, what is known in some lit. as maneuvering LEFs.
– user14897
May 2 at 16:49

The LEF's (leading edges in front of the wings) go down symmetrical, there is a asymmetry brake system built in to prevent that asymmetrical deflection if more than a few degrees (5 degrees) difference is sensed, immediately. Flaps or flaperons from the behind edge of the wings do that and some jets have stabilators moving like scissors to do this. At slower speed the amount of angle of attack is huge and this provides lift. At high speed angle of attack is small and LEF's in down position create drag more than normal values because the airflow is tangent not in line with the leading edges angle of deflection .

• Suggestion: delete sentence "You don't roll if the LEF's are asymmetrical." It doesn't seem to add anything, and is potentially confusing. If the LEF's were asymmetrical, you very well might roll. Apr 4 at 18:13
• They're symmetric on the F-16, but not on the F/A-18, for example.
– user14897
May 2 at 16:57
• True when you roll the F-18 SuperHornet. I assume that is valid for legacy Hornet to. May 3 at 17:46